1. Field of the Invention
The present invention relates to a process for preparing an aqueous polymer dispersion having a polymer solids content ≧50% by weight by free-radically initiated aqueous emulsion polymerization of ethylenically unsaturated compounds (monomers), which comprises    a) charging a mixture comprising
portion 1of deionized water,from 0.001 to 0.5 part by weightof polymer seed havinga weight-average particlediameter ≦100 nm,from 0.0001 to 0.1 part by weightof dispersant, andfrom 0.5 to 10 parts by weightof monomers     to a polymerization vessel and polymerizing the monomers to a conversion ≧80% [stage A] and    b) subsequently supplying to the resultant reaction mixture under polymerization conditions a monomer emulsion comprising
portion 2of deionized water,from 0.1 to 5 parts by weightof dispersant, andfrom 90 to 99.5 parts by weightof monomers     [stage B], where    c) the total amount of monomers used for the polymerization is 100 parts by weight and the total amount of deionized water is ≦100 parts by weight.
The present specification further relates to the use of the aqueous polymer dispersions obtained by the process of the invention as binders, for modifying cement formulations and mortar formulations, and as adhesives, especially pressure sensitive adhesives.
2. Description of the Background
Like polymer solutions when the solvent evaporates, aqueous polymer dispersions, when the aqueous dispersion medium evaporates, display the property of forming polymer films, which is why aqueous polymer dispersions find application in a variety of ways as binders, for paints, for example, or for compositions used to coat leather, paper or polymer films.
Aqueous polymer dispersions with a high polymer fraction are of particular advantage in that, on the one hand, their relatively low proportion of aqueous dispersion medium reduces the energy required for evaporating it, for the purpose of forming films or for producing polymer powders, for example, and on the other hand, the useful polymer can be stored and transported using a relatively small amount of aqueous phase as carrier medium.
However, there is a disadvantage in that, as the volume concentration of the polymer increases (U.S. Pat. No. 4,130,523), there are problems associated with the preparation of aqueous polymer dispersions. For instance, on the one hand, there is an increase in the flow resistance (the viscosity) which makes it more difficult both to dissipate the heat of reaction and to process the aqueous dispersion, and on the other hand there is an increase in the propensity of the dispersed polymer particles to aggregate for reasons of thermodynamic stability. The resulting flocs [a) microflocs or bits; not normally removable by conventional filtration; b) macroflocs or coagulum; normally removable by customary filtration] lead in particular to defects in the formation of films from the aqueous polymer dispersions, and are therefore generally unwanted.
According to studies into the flow resistance of aqueous polymer dispersions, those having a bimodal or broad size distribution (polydispersity) of the dispersed polymer particles for a given solids content are generally less resistant to flow than those having a narrow size distribution (which are in the extreme case monodisperse). Moreover, coarse aqueous polymer dispersions present a lower resistance to flow than do fine aqueous polymer dispersions, given the same solids content.
DE-A 19633967 discloses readily filterable and deodorizable, aqueous, pressure sensitive adhesive dispersions having solids contents of more than 65% by weight which are prepared by a special monomer feed process. Over the course of a quarter of an hour to an hour, from 1 to 10% by weight of the total monomer amount is metered in the form of a monomer emulsion at an increasing rate, after which the remainder of the monomer emulsion is run in continuously in the standard way.
DE-A 19628142 discloses aqueous polymer dispersions having solids contents ≧50% by weight and a bimodal particle size distribution. They are prepared by adding a monomer miniemulsion (average droplet size from 10 to 500 nm) during the polymerization. On an industrial scale, however, preparing a monomer miniemulsion requires additional costly homogenizing units, such as disperser discs or ultrasound devices, for example.
DE-A 4307683 describes an expensive and complex process for preparing an aqueous polymer dispersion having a solids volume concentration of ≧50%. A characteristic of this process is that an initial polymer dispersion in which at least 99.6% by weight of the polymer particles have a diameter ≧90 and ≦500 nm is introduced to the polymerization vessel as an initial charge before the actual emulsion polymerization. The subsequent emulsion polymerization is characterized by a two-stage monomer feed. The aqueous polymer dispersion which results has a polymodal particle size distribution. Additionally, DE-A 4307683 provides a good overview of the further state of the art.
DE-A 3147008 discloses a process for preparing bimodal aqueous polymer dispersions having a solids content ≧50% by weight, where prior to the actual emulsion polymerization a blend of two polymer dispersions differing in average particle size is added to the reaction vessel. A disadvantage of the process is the need to hold two aqueous polymer dispersions.
DE-A 3319340 describes a process for preparing bimodal or polymodal aqueous polymer dispersions with a solids content of up to 59% by weight. A characteristic of the process is that during the emulsion polymerization it is necessary to supply a seed latex to the reaction mixture before the monomer conversion exceeds 40%.
EP-A 784060 discloses a process for preparing bimodal aqueous polymer dispersions having a solids content ≧67% by weight and a viscosity ≦2000 mPa*s. The process is distinguished by the introduction as initial charge to a reaction vessel at polymerization temperature of an aqueous initiator solution with no emulsifier and the subsequent supply of a monomer mixture containing emulsifier. An essential feature of the process is that, after from 40 to 60% by weight of the monomer mixture have been metered in to the polymerization mixture, an additional quantity of emulsifier is added to the polymerization mixture.
A disadvantageous feature of the aforementioned processes is that in many cases, while giving good results on the laboratory scale, they are too complex or insufficiently reproducible for the industrial scale and, furthermore, exhibit long cycle times.